Where can one find such a soft magnetic material?

Question

Physics

I need to find a material with certain parameters of magnetic properties. The most important requirement, the value of the magnetic viscosity is 0.5-1 ms, meaning the constant magnetic viscosity used in the expression for changing the magnetization of a ferromagnet.
The change in magnetization ΔJ depending on time t is described as:
ΔJ(t)=[J(t)– J0]=[J∞ – J0]*[1exp(t/τ)],
where J0 and J∞ are respectively immediately after the change in the strength H of the magnetic field at the moment t = 0 and after the establishment of a new equilibrium state.
The saturation induction value of a ferromagnet should be minimal.
The demagnetization time of a ferromagnet is not critical.
The material is needed to create a toroid that will work in my device.